Earlier today a Reddit user posted a photo of a female coworker under the headline: "This employed (sic) at my University has five fingers per hand...but no thumbs."

(Asked about the typo, OP said, "sadly my left thumb cannot reach the 'e' key." Seems like an extra finger would have come in handy.)

It didn't take long for the sideshow audience to upvote the post to the front, which naturally brought the "med students" out from under their Gray's Anatomy:

Med student here. Just learned about this the other day. Its called "triphalyngeal thumb." You know how there are three joints in your fingers, but only two in your thumb? An error in embryological development causes the thumb to have three joints.

There are different classifications of triphalyngeal thumb. This case appears to not be opposable. Normally there is loss of function because the thumb has the same functions as a finger, so no grasping. Only flexion, extension, adduction and abduction, no "crossing" or "grasping" like a normal thumb. Here is a better picture.